1. Field of the Invention
The present invention relates to a method for removing dense non-aqueous phase liquids in porous media by the use of surfactant-enhanced aquifer remediation. More particularly, the present invention involves the use of solutions which produce generally neutral buoyancy during the cleanup of dense non-aqueous phase liquids to control vertical migration and thus enable surfactant-enhanced aquifer remediation in aquifers lacking a clay bottom layer or other form of aquiclude to prevent downward migration of the solubilized dense non-aqueous phase liquids. Additionally, such a method of aquifer remediation can also be used on aquifers having an aquiclude.
2. State of the Art
For many years little care was taken in the handling of organic solvents and other materials which were used in industry and at government installations, such as military bases. Because of poor handling techniques and occasionally intentional dumping, many industrial sites and military bases now have contaminated areas containing relatively high concentrations of the contaminants. Chlorinated solvents such as trichloroethylene and perchloroethylene and other types of organic liquids are common at such sites and, if not removed, can filter down into groundwater supplies, rendering the water unfit for consumption and other uses.
A variety of techniques have been used to promote the removal of such chemical contaminants both from the soil and from the groundwater. The principle method of ground-water remediation currently used where dense, non-aqueous phase liquids are involved utilizes what is commonly referred to as "pump and treat". In such a method wells are drilled into the contaminated area and contaminated groundwater is pumped above the surface, where it is treated to remove the contaminants.
The limitations of the pump and treat method have been documented in articles such as Mackay, D. M. and J. A. Cherry, Groundwater Contamination: Pump and Treat Remediation, 23(6) ENVIRON. SCI. TECHNOL. 630-36, 1989. The article concludes that pump and treat remediation can only be relied upon to contain ground-water contamination through the manipulation of hydraulic gradients within an aquifer. The reasons for the failure of pump-and-treat to decontaminate aquifers are rooted in the limited aqueous solubility of many dense, non-aqueous liquids in groundwater and other processes involving contaminant desorption and diffusion. Because of the low aqueous solubility of most dense non-aqueous liquids, their removal by groundwater extraction requires exceptionally long periods of time.
Due to the general impractability of the pump and treat method, considerable attention has been paid recently to methods which utilize a process commonly referred to as enhanced solubilization. Such a method uses micellar surfactant solutions to increase the effective solubility of the dense non-aqueous contaminants to accelerate the rate of removal. The mechanism for solubilization displayed by surfactants arises from the formation of microemulsions by the surfactants, water, and the solubilized dense non-aqueous liquid molecules. For example, FIG. 1A shows a table of solubilization of perchloroethylene (PCE) by various nonionic and anionic surfactant solutions. The results indicated that even dilute surfactant solutions can significantly increase the aqueous solubility of perchloroethylene.
The effectiveness of the surfactant enhanced aquifer remediation or solubilization method was studied in a major field test on in-situ solubilization by Professor J. C. Fountain of the State University of New York at Buffalo. Fountain, J. C. et al., A Controlled Test of Surfactant-Enhanced Aquifer Remediation, 34(5) GROUNDWATER 910-916 (1996). The test consisted of the controlled release of 231 liters of perchloroethylene into a shallow sand aquifer having a clay aquiclude formed, thereunder.
Prior to conducting the test, an estimated 70 liters of PCE was excavated from the test bed. Free phase extraction and water-flooding removed another 49 liters of the contaminant. A 2 percent mixture of nonylphenol ethoxylate and phosphate ester of an alkylphenol ethoxylate was injected into the aquifer through 5 wells on one side of the spill, and removed by five wells on the opposing side. The surfactant flood resulted in extraction of another 67 liters of PCE. Extraction was primarily due to solubilization of the contaminant, though some mobilization of PCE occurred as well.
Excavation of the site suggested that about 16 liters of perchloroethylene was not removed by the surfactant flood. This volume was found pooled above the clay aquiclude at the bottom of the aquifer.
Others have tested surfactants which will produce a sufficiently low interfacial tension to remove substantially all of the dense non-aqueous phase liquid contaminant. Tests utilizing a microemulsion with an interfacial tension (IFT) of less than 0.01 dyne/cm have removed all measurable dense non-aqueous phase liquids (DNAPL) in the test section. Reduction of IFT between the DNAPL and microemulsion results in a combination of mobilization and solubilization of the contaminants. This application of surfactant-enhanced aquifer remediation is very effective, but nevertheless does not address the concern over uncontrolled, vertical migration of the free-phase DNAPL (during mobilization), or of a microemulsion containing the DNAPL compounds (during solubilization) in aquifers not underlain by an aquiclude. The technology described herein is designed to address such concerns over vertical migration. In that light, then, the application of surfactant-enhanced aquifer remediation described here is one in which only solubilization of the DNAPL compounds occur. That is, this technology is specifically designed to ensure that no mobilization of the DNAPL occurs.
Referring now to FIG. 1C, there is shown a cross sectional view of the application of the surfactant-enhanced aquifer remediation system at work. A well 10 is formed on one side of the spill area 20 and is configured for flooding the spill area with a surfactant, such as one of those described above. As the dense non-aqueous phase liquid is solubilized, it is carried to an extraction well 30 disposed on the opposite side of the spill area 20. While the solubilized dense, non-aqueous phase liquid moves through the aquifer, it may continue to migrate vertically due to the microemulsion's density relative to the surrounding groundwater. Generally, a sufficiently thick aquiclude 40 such as a clay or shale layer will preclude its continued migration. Thus, despite such vertical migration of the dense, non-aqueous liquid contaminants, over time the entire spill area is substantially cleaned of the contaminant if wells are screened all of the way to the aquiclude.
One serious drawback with the surfactant-enhanced aquifer remediation is that the vertical mobility of the solubilized dense non-aqueous phase liquid substantially requires that an aquiclude be present to catch any solubilized contaminant which migrates sufficiently vertically.
Many aquifers, however, lack such an aquiclude. If the traditional surfactant-enhanced aquifer remediation method were to be used with an aquifer lacking an aquiclude, there is a significant risk that the solubilized dense non-aqueous liquid will spread vertically and contaminate an increasingly large volume.
Thus, there is a need for an improved method of extracting dense non-aqueous phase liquids from aquifers. Such a method must minimize vertical mobility of the solubilized material, while providing relatively quick clean-up of the contaminants.